Apparatus for detecting magneto-optical anisotropy

ABSTRACT

This invention relates to an apparatus for detecting magneto-optical anisotropy which can be utilized as an information reproduction apparatus for detecting information recorded on a magnetic medium. The apparatus consists of a light source (10), a magneto-optically anisotropic medium (20), a polarization analyzer (33A) and a light detector (34). To avoid the information reproduction by means of the linearly polarized light as in the prior art, the apparatus of the present invention reproduces the information using the light source (10) which generates the elliptically polarized light. This makes it possible to reproduce the information having a high signal-to-noise ratio.

TECHNICAL FIELD

This invention relates to an apparatus for detecting magneto-opticalanisotropy of a magnetic medium. The magneto-optical anisotropy dealtwith in this invention represents the property such as double refractionand circular dichroism, that changes the linearly polarized light intothe elliptically polarized light or causes a change in the ellipticityof the elliptically polarized light. The term "elliptically polarizedlight" used in this specification includes the circularly polarizedlight as a special case where the ellipticity takes a specific value.

BACKGROUND ART

In an apparatus used conventionally, as a magneto-optical anistropydetector of a magnetic medium, for reading out information recorded inaccordance with the magnetized state of the magnetic medium, when thelinearly polarized light passes through the magnetic medium or isreflected on the surface of the magnetic medium, the incident linearlypolarized light is slightly converted into the elliptically polarizedlight and the principal axis of the ellipse slightly rotates from thepolarizing direction of the incident linearly polarized light.Conventionally, the rotational angle of the principal axis ofpolarization is measured using a polarizing element and a polarizationanalyzer, but this method involves the drawbacks in that the apparatusis complicated and a signal-to-noise ratio is low.

DISCLOSURE OF INVENTION

The present invention provides an apparatus for detectingmagneto-optical anisotropy which employs the elliptically polarizedlight in order to eliminate the abovementioned problems with theconventional magneto-optical anisotropy detector and which has a widerrange of application and higher performance by measuring the change ofthe ellipticity of the elliptically polarized light.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are block diagrams useful for explaining the fundamentalconstruction of the present invention;

FIG. 3 is a block diagram showing the construction of an embodiment ofthe apparatus for detecting the magneto-optical anisotropy of thepresent invention;

FIG. 4 is a schematic view of the polarized state of light useful forexplaining the operation of the embodiment shown in FIG. 3;

FIG. 5 is a block diagram showing the construction when the presentinvention is applied to a magnetic disc;

FIG. 6 is a block diagram showing the construction of another embodimentof the apparatus for detecting the magneto-optical anisotropy of thepresent invention;

FIG. 7 is a block diagram showing the construction of still anotherembodiment of the apparatus for detecting the magneto-optical anisotropyof the present invention;

FIG. 8 is a block diagram showing the construction when the presentinvention is applied to an optical disc;

FIGS. 9 and 10(a)-(c) are schematic views useful for explaining theoperation of the apparatus shown in FIG. 8;

FIG. 11 is a block diagram showing the construction of still anotherembodiment of the apparatus for detecting the magneto-optical anisotropyof the present invention;

FIGS. 12(a) and 12(b) schematic views showing the principal portionsforming the embodiment shown in FIG. 11;

FIGS. 13(a) and 13(b) are schematic views useful for explaining theoperation of the embodiment shown in FIG. 11;

FIG. 14 is a block diagram showing the construction of still anotherembodiment of the apparatus for detecting the magneto-optical anisotropyof the present invention;

FIG. 15 is a schematic view showing the axis of polarization forexplaining the operation of the embodiment shown in FIG. 14; and

FIG. 16 is a schematic view showing another construction of theprincipal portions of the embodiment shown in FIG. 14.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be explained in detail withreference to embodiments thereof.

FIGS. 1 and 2 illustrate the fundamental construction of the presentinvention. More specifically, FIG. 1 shows the apparatus of atransmission type in which the ray of light passes through a medium andFIG. 2 shows the apparatus of a reflection type in which the ray oflight is reflected by the medium. In both drawings, the same referencesymbols represent the same parts or the same constituent members.

In FIGS. 1 and 2, reference numeral 10 represents a light source for thecircularly polarized light or for the elliptically polarized light; 20is a magneto-optically anisotropic medium; 30 is a polarization analysismeans; and 40 is optical anisotropy-inducing means. If themagneto-optically anisotropic medium 20 is a substance having by itselfnatural anisotropy due to its magnetic structure or the like and if itis desired to observe such a substance, the optical anisotropy-inducingmeans as the additional means are not always necessary.

In most cases, the magnetizing direction of the magneto-opticallyanistropic medium 20 is parallel or perpendicular to the surface of themedium. However, the present invention can be applied even when themagnetizing direction is an optional direction other than theabovementioned directions.

An embodiment of the apparatus in accordance with the present inventionis shown in FIG. 3. In this drawing, the ray of light of the lightsource 10 emitting the elliptically polarized light is incident to themagneto-optically anisotropic medium 20 and its transmission light isdetected by a light receiver 34 via a polarization analyzer 33A. Themagnitude of the optical anisotropy of the medium 20 appears as a changeof the quantity of light received by the light receiver 34. This changeof the quantity of the light can be detected further accurately byemploying a differential means, or a means comprising detecting a partof the ray of light of the light source 10 by means of a beam splitter50, a polarization analyzer 33B and a light receiver 35, comparing thequantity of the light detected thereby with that of the light receiver34 using a comparator 60 and producing the output at an output terminalA.

The characterizing feature of the present invention lies in that a lightsource for the elliptically polarized light of which ellipticity is notzero, that is, a light source not for the linearly polarized light, isemployed as the light source. Even when the light source for thelinearly polarized light is employed as the light source, however, thesimilar object can be accomplished by adding means for generating theelliptically polarized light consisting of a phase plate or a doublerefraction element, a circular dichroism element and the like to theoptical path in addition to the abovementioned constituent elements.

The inserting position of the phase plate or the circular dichroismelement in the latter case is between the light source 10 and the medium20 as represented by 90A or between the medium 20 and the polarizationanalyzer 33A as represented by 90B in FIG. 3.

Owing to the abovementioned construction, the apparatus of the presentinvention is characterized in that the apparatus makes it possible todetect not only the magnitude but also the polarity of themagneto-optical anisotropy of the magnetic medium with a high level ofsensitivity. As a definite example, it will now be assumed that themedium 20 is one that exhibits magneto-optical circular dichroism suchas a thermo-magnetic recording medium, and that the magnetizingdirection is parallel to the optical path. If this medium is notpresent, the ray of light incident to the polarization analyzer 33A iselliptically polarized light of a certain ellipticity as represented byan ellipse a in FIG. 4. In FIG. 4, however, the ordinate Y and theabscissa X represent the amplitude of the light in the electric field intwo directions crossing at a right angle with each other. Owing to thepresence of the medium 20, change occurs in the ellipticity so that theray of light becomes elliptically polarized light as represented by anellipse b of FIG. 4, for example. Accordingly, if the polarizingdirection of the polarization analyzer is the direction of the X axis,this change in the ellipticity depends upon the magnitude ofmagnetization. If magnetization of the medium 20 inverses, theelliptically polarized light changes into the shape such as shown by anellipse c in the drawing. When compared with the original ellipticallypolarized light a, the intensity of the light in the direction of the Xaxis is decreased. In this manner, it becomes possible to simultaneouslydetect the magnitude and polarity of the magnetic circular dichroism bymeans of the change of the ellipticity.

What is of importance in the abovementioned embodiment is that theellipticity of the elliptically polarized light incident to the mediummust be set to a value greater than a certain value. Otherwise, theminimum state of the ellipticity represented by the ellipse c of FIG. 4becomes extremely approximate to the linearly polarized light and thepolarization component in the direction of the X axis becomes extremelysmall. Under such a state, the signal output is small and issubsceptible to noise disturbance due to incompleteness of the lightdetecting element or to the dark current of the light receiver. Thesignal output, or the change quantity of the ellipticity, increases withthe ellipticity in such a range where the ellipticity of the incidentelliptically polarized light is not very great. Irrespective of themagnetization state of the magnetic medium, therefore, the ellipticityof the incident elliptically polarized light must be set so that theelliptically polarized light is constantly incident to the polarizationanalyzer. This can be accomplished by sufficiently inclining theprincipal axis of the aforementioned phase plate relative to thepolarizing direction of the incident light to the phase plate. In thecase of the abovementioned circular dichroism element, the arrangementmust be made so that the difference of the adsorption factors to theright and left circularly polarized light becomes sufficiently large.

When setting is made in the abovementioned manner, a signal-to-noiseratio can be improved drastically.

So long as the ellipticity is not extremely small, the rotation of theprincipal axis of the ellipse due to the magneto-optical effect can beneglected.

If, in FIG. 3, the medium 20 is constructed by a magnetic thin filmformed on a substrate so as to represent information by means of theintensity of magnetization or its polarity, it can be utilized as aninformation recording medium corresponding to a so-called magnetic disc.FIG. 5 shows its embodiment. The optically anisotropic medium isconstructed by forming the magnetic thin film 22 on the substrate 21.The ray of light of the light source for the linearly polarized laser isconverted into elliptically polarized light or circularly polarizedlight by a double refraction element 90A, passes through a beam splitter50, is reflected by the magnetic, optically anisotropic medium and isdetected by the receiver 34 via the beam splitter 50 and thepolarization analyzer 33A. The ray of light reflected by the medium 20is changed into the elliptically polarized light of the ellipticityvarying from that of the incident light due to the magnetic Kerr effect.This change is detected by the abovementioned method.

FIG. 6 shows another embodiment of the present invention. The lightsource is a Zeeman laser. The Zeeman laser has a laser 11 and a magneticfield generation apparatus 12 and generates right circularly polarizedright and left circularly polarized light having a different frequencyfrom each other. This relies upon the Zeeman effect and is a knowntechnique. According to the heretofore known Zeeman laser technique, aresonator length is automatically adjusted so that the intensity of theright and left circularly polarized light becomes equal to each other.According to this arrangement, a composite wave of the right and leftcircularly polarized light is linearly polarized light and thepolarizing direction of the linearly polarized light rotates with thetime. This linearly polarized light is converted into the ellipticallypolarized light by means of an elliptically polarized light generator90A. This generator is produced by forming a magnetic, circular dichroicthin film 91A on a substrate 92A, for example. The optically anisotropicmedium is produced by forming a thin layer 22 of a thermomagneticrecording substance on a substrate 21 and is actuated for rotation by amotor 23.

The characterizing feature of the apparatus of this embodiment residesin that since the ray of light generated from the Zeeman laser consistsof plural oscillation lines having varying frequencies from one another,they generate beat due to their mutual interference so that the ray oflight incident to the light receivers 34 and 35 is subjected to theintensity-modulation. In view of this fact, in the embodiment shown inFIG. 6, band-pass filters 36 and 37 are added behind the light receivers34, 35. The center frequency of the band-pass filters is tuned with theabovementioned modulation frequency and their frequency is set so as tobe higher than the frequency of the signal generated from the recordingmedium 20. The band width of the filter is set so as to be equal to, orhigher than, the band width of the abovementioned signal. The apparatusof this embodiment is characterized in that a signal-to-noise ratio canbe improved by suitably selecting the modulation frequency of thesignal.

The Zeeman laser employed in the abovementioned embodiment is theheretofore known type in which the intensity of the right and leftcircularly polarized light is equal to each other and the composite wavebecomes linearly polarized light. For this reason, the embodiment needsthe elliptical polarizer as represented by 90A in FIG. 6. As an exampleto improve this point, an example of the Zeeman laser construction inwhich the laser light itself becomes elliptically polarized light isshown in FIG. 7. A magnetic field parallel to the optical axis isimpressed upon a laser active substance 110 by means of a magnetic fieldgenerator 12. A light resonator is formed by two reflecting mirrors 13Aand 13B. The output light of the Zeeman laser constructed in this mannerbecomes the right and left circularly polarized light having a varyingfrequency from each other as described already, but the intensity of theright and left circularly polarized light in the Zeeman laser used inthe present invention is controlled in the following manner using acontrol system 14 encompassed by dotted lines in FIG. 7.

Namely, a part of the output light is separated by a beam splitter 141and is passed through a quarter wavelength plate 142 whereby the rightand left circularly polarized light becomes two rays of linearlypolarized light that cross each other at a right angle. They areseparated by a polarization analyzer 143 and their intensity aremeasured by light receivers 144A and 145B and compared by a comparator145. If the difference, of the right and left circularly polarized lightis not equal to a predetermined reference value applied from a referencevalue input terminal 147, deviation from the predetermined referencevalue is detected using a comparator 146 and a resonator length isadjusted using a driving mechanism 148 so that they become equal to eachother. When the reference value is set to zero, the intensity of theright and left circularly polarized light becomes equal to each otherand the composite wave becomes linearly polarized light as describedalready. When a reference value which is not zero is applied, there isobtained elliptically polarized light having elliptically in accordancewith the reference value. In the elliptically polarized light thusobtained, the direction of its principal axis rotates with the time inthe same way as the linearly polarized light shown in FIG. 6.Accordingly, it is possible to detect the change of the ellipticityusing the same optical system as shown in FIG. 6.

That is to say, the elliptical polarization Zeeman laser shown in FIG. 7is used as the light source 10 and the elliptical polarization generator90A is removed because it is not necessary. According to thisarrangement, the rays of light incident to the light receivers 34 and 35are subjected to the intensity modulation and their amplitudecorresponds to the ellipticity. Accordingly, the modulated amplitude ofthe ray of light incident to the light receiver 34 corresponds to theellipticity of the light passing through the recording medium 20 andthat of the light incident to the light receiver 35 corresponds to theellipticity of the light emitted from the light source. For this reason,it is possible to detect the change of the ellipticity due to theoptically anisotropic medium in the same way as in the embodiment shownin FIG. 3.

FIG. 8 shows still another embodiment of the present invention. Thisembodiment relates to an apparatus for recording and reproducinginformation on an optical disc using the magnetooptical anisotropy inthe transverse magnetic field. The information disc is produced byforming the magnetic thin film 22 on the surface of the glass substrate21 and is driven for rotation by the motor 23. A write magnetic head 41is disposed close to the magnetic thin film 22 and is excited by a powersource 42. The ray of light of the linearly polarized light laser 11 isconverted into the circularly polarized light by a 1/4-wavelength plate90 and into the elliptically polarized light as it passes through themagnetic thin film 22, causes rotation in the orientation of theprincipal axis of the ellipticity polarized light as it further passesthrough a Faraday rotation element 31 and is then separated into two,mutually orthogonal, linearly polarized light by a polarization analyzer33. Each polarized light component is incident to the light receiver 34,35 and its intensity is compared with that of the other by a comparator60 so that a signal corresponding to the difference of the intensityappears at the output terminal A. The Faraday rotation element 31 isexcited by the power source 38 and its excitation current is measured byan excitation current meter 39. The measured value appears at the outputterminal C. The signal appearing at the output terminal A of thecomparator 60 is fed back to the input terminal B for controlling theexcitation current of the power source 38. As magneto-optic recordingmedia, various magnetic materials are known in the art. As the Faradayrotation element, there are also known various substances havinglight-transmitting property and a large Verdet constant.

Next, the principle of operation of the apparatus of the presentinvention will be explained. Initially, the method of writinginformation will be described. As viewed from the directionperpendicular to the disc surface, the magnetic head 41 consists ofmagnetic poles 41XA and 41XB for generating the magnetic field in the Xdirection and magnetic poles 41YA and 41YB for generating the magneticfield in the Y direction as shown in FIG. 9. These poles are excited bypower sources 42X and 42Y, respectively. Within the disc surface, themagnetic thin film 22 is magnetized in an optional direction byregulating the ratio of the X direction excitation current and the Ydirection excitation current. The size of the range to be magnetized isdetermined by the width of the gap of the magnetic head. Discriminationof the information, that is, 0 and 1 in the digital recording or asignal level in analog recording, is made by the azimuth ofmagnetization. In other words, if a rated input signal level impressedby the input terminal In is selected within the range of -π/4<θ<π/4, sinθ and cos θ are generated by a function generator, X-directionY-direction magnetic fields are generated by excitation currentsproportional to them, and the azimuth θ of the composite magnetic fieldrelative to the X axis becomes equal to the input signal. Largequantities of information can be recorded by aligning minute magnetizedregions while the disc is being rotated. As the recording method, theheretofore known thermo-magnetic recording methods (Curie pointrecording, compensation temperature recording, etc.) may be employed.

Next, a reproduction method of reading out the recorded information willbe described. As shown in FIGS. 8 and 9, the X and Y axes are positionedwithin the surface of the optical disc while the Z axis is positionedperpendicularly to the surface, that is, in the direction of the opticalpath. The orientation of the polarization analyzer 33 is arranged insuch a manner that the X direction linearly polarized light (pcomponent) is incident to the light receiver 34 while the Y directionpolarized light (s component) is incident to the light receiver 35.

As the circularly polarized light generated by the light source 10passes through the magnetic thin film 22, it is converted into theelliptically polarized light due to the magnetooptical effect (Voigteffect, in this case). As shown in FIG. 10(a1), if the magnetizingdirection M is within the disc surface and is +45° relative to the Xaxis, the elliptically polarized light after the passage through themagnetic thin film is polarized in the orientation with the X- andY-axes being the principal axes. Accordingly, the difference ΔI=I_(X)-I_(Y) between the intensity of the X direction polarization componentI_(X) and the intensity of the Y direction polarization component I_(Y)scores the minimal value (negative value) in the case of the embodimentshown in this drawing. This difference signal appears at the outputterminal A of FIG. 8, is applied to the control input terminal B of thepower source 38, the control signal regulating the exciation current ofthe Faraday rotation element 31. As a result, as shown in FIG. 10(a2),the ray of light after the passage through the Faraday rotation element31 has the azimuth of the principal axis of the elliptically polarizedlight rotated by -45° and stops rotating when I_(X) and I_(Y) becomeequal to each other.

To sum up the abovementioned, when the azimuth of the magnetization isinclined by +45° relative to the X axis as shown in FIG. 10(a1), theFaraday rotation angle is such an angle as to compensate for theinclination, that is to say, -45°, and the Faraday rotation elementexcitation current scores a large negative value.

Next, when the orientation of magnetization M coincides with thedirection of the X axis as shown in FIG. 10(b1), the principle axis ofthe elliptically polarized light coming out from the magnetic thin film22 is, as shown in FIG. 10(b2), inclined by 45° from the X axis. Inconsequence, the I_(X) and I_(Y) are equal to each other and thedifference signal becomes zero. Hence, the Faraday rotation angle aswell as the excitation current also become zero. Finally, as shown inFIG. 10(c1), when the direction of magnetization M is inclined by -45°from the X axis, the principal axis of the elliptically polarized lightcoincides with the X and Y axes, as shown in FIG. 10(c2), whereby thedifference signal ΔI between I_(X) and I_(Y) scores a large positivevalue and the Faraday rotational angle compensating for the signalbecomes +45° with the Faraday rotation element excitation current beinga positive maximum value. In the intermediate state other than theabovementioned state, both Faraday rotation angle and excitation currentscore intermediate values, respectively.

The above may be summarized as follows. The orientation of the principalaxis of the elliptically polarized light after passing through themagnetic thin film 22 depends upon the orientation of the magnetizationM and its magnetization azimuth can be determined by the magnitude andpolarity of the excitation current of the Faraday rotation element 31.Accordingly, in recording, if the orientation of the magnetization M isvaried during recording in accordance with 0 and 1 of the input digitalsignal or with the height of the level of the input analog signal, therecorded content can be reproduced optically. In this case, the originalsignal can be reproduced as the magnitude and sign of the excitationcurrent of the Faraday rotation element in accordance with the azimuthof the magnetization M. The reproduction signal is produced as output atthe output terminal C via the measuring equipment 39 of the excitationcurrent of the Faraday rotation element.

FIG. 11 shows still another embodiment of the present invention. Thisembodiment is of the reflection type construction shown in FIG. 2. Theray of light from the light source 10 for the circularly polarized lightor the elliptically polarized light is incident to the magnetic thinfilm 22 through a condenser lens 15, a reflecting mirror 18 having ahole and a focusing lens 16. The ray of light reflected on the surfaceof the thin film is focused at an image-forming point F and passesthrough the lens 16, the reflecting mirror 18, the lens 19 and thepolarization analyzer 33 whereby the p component reaches the lightreceiver 34 while the s component reaches the light receiver 35. Asviewed from such a direction that the rays of incident light to thelight receivers 34 and 35 are also incident to the eyes of the observer,the light receivers 34 and 35 have a quartered construction such asshown in FIGS. 12(a) and 12(b) (with the divided ranges represented byA, B, C and D). As can be appreciated by tracing the rays of light,among the rays of light incident to the magnetic thin film 22, the pcomponent of the ray of light A of which incident surface coincides withthe sheet surface of the drawing reaches the light receiving element 34Awhile its s component reaches the light receiving element 35A. The pcomponent of the ray of light B reaches the light receiving element 34Bwhile its s component reaches the light receiving element 35B.Similarly, as to the rays of light whose incident surface isperpendicular to the sheet surface of the drawing, the ray of lightincident from one side and reflected to the other side is incident tothe light receiving elements 34C and 35C while the ray of light incidentfrom the other side of the sheet surface and reflected to this side isincident to the light receiving elements 34D and 35D.

It will be assumed here that the magnetic thin film 22 is magnetized bya magnetic head 41 in the direction parallel to the sheet surface (+xdirection) as indicated by arrow, and that the incident, ellipticallypolarized light to the magnetic thin film 22 is elliptically polarizedlight of which x-direction polarization component parallel to the sheetsurface is small and of which y component perpendicular to the sheetsurface is by far greater than the x component. Under such a condition,there occur a longitudinal kerr effect to the rays of light A and B anda transverse effect to the rays of light whose incident surface isperpendicular to the sheet surface. As a result, the intensity of theray of light incident to each light receiving element (p component forthe light detection element 33) of the light receiver 34 changes in thefollowing manner. That is to say, the intensity of the ray of lightincident to each light receiving element is represented as shown in FIG.13(a) with a reference set under a condition where the magnetization ofthe magnetic thin film 22 is zero. Thus, the light receiving ranges A,B, C and D of the light receiver 34 correspond to +, 1, 0 and 0,respectively, where + represents the increase, - does the decrease and 0does no change or a slight change.

Under this state, when the magnetization of the magnetic thin film 22inverses and faces in the negative direction of the x axis, the lightintensity of each light receiving element inverses from + to - and viceversa with 0 remaining unaltered. In this manner, it is possible toidentify by means of the intensity distribution pattern on the quarteredlight receiver whether the magnetization of the magnetic thin film facesin the positive direction of the x axis or in its negative direction.For some magnetic materials it may happen that the increase and decreaseof the light intensity incident to the light receiving elements assumethe opposite polarities to those mentioned above but there is noessential difference in the principle of identifying the magnetizingdirection.

Next, consideration is made when the magnetic thin film 22 is magnetizedin the direction perpendicular to the sheet surface of the drawing. Inthis case, the light intensity incident to each light receiving elementchanges as shown in FIG. 13(b). Namely, the light receiving ranges A, B,C and D of the light receiver 34 are 0, 0, + and -, respectively. Whenthe magnetization inverses under this state, the abovementioned + and -inverse with each other.

To sum up, it is possible to record and reproduce four-value informationon one minute spot. This four-value information corresponds to +x, -x,+y and -y depending on the orientation of the magnetization. Each of themagnetized states produces a varying intensity pattern on the quarteredlight receiver when reproduced optically. The technique described inthis embodiment provides a high density recording/reproduction techniquerepresenting binary or non-binary information in accordance with theorientation of the magnetization. Incidentally, the light receiver 35 inthis embodiment functions to provide a reference value of the lightintensity in the same way as in the embodiment shown in FIG. 3.

FIG. 14 shows still another embodiment of the present invention. Thelight source is a helium-neon laser or a semiconductor laser, forexample. The state of polarization is linearly polarized light parallelto the sheet surface of the drawing, that is, the linearly polarizedlight in the x axis direction. This polarized light is hereinaftercalled "p polarized light". The ray of light from the light source 11 ismade substantially parallel by a collimator lens 15 and passes through abeam splitter 50. This beam splitter is constructed by a transparentglass sheet, for example, and most of the p component pass through thissplitter. Next, the laser light is incident to a polarizing prism 33.Most of the p component pass through this prism and are converted intothe circularly polarized light or elliptically polarized light as theypass through a phase plate 90. The ray of the polarized light is thenconverged by a lens 16 and is then incident to a magnetic recordingdisc. The magnetic recording disc is produced by depositing the magneticthin film 22 onto the surface of a glass or plastic substrate 21 and isdriven for rotation by a motor 23. Recording of information is effectedby locally magnetizing the magnetic thin film using a magnetic head 41.In this instance, 0 and 1 of the digital information can bedistinguished whether the magnetizing direction, which is perpendicularto the disc surface, for example, faces the +z direction or the -zdirection. In practice, however, it is desirable for realizing the highdensity that a recording area per bit information is as small aspossible. Hence, it is more advisable to employ the thermomagneticrecording technique. In this technique laser beam is focused onto thedisc surface which is more intense than in reproduction and themagnetized state is locally changed by heating a magnetic substance to atemperature higher than a certain critical point. This technique is wellknown in the art of the magneto-optic field.

Various substances are known for the magnetic thin film. Agadolinium-cobalt film formed by sputtering may be used, for example.This film is magnetized by the abovementioned method and theelliptically polarized light or the circularly polarized light isirradiated upon the film by the abovementioned method. The lightreflected by the magnetic film passes again through the lens 16 and thephase plate 90 and is again incident to the polarizing prism 33. Thisre-incident light is in the polarized state different from the pcomponent emitted from the laser, owing to the action of the phase plate90 and to the magnetic Kerr effect on the magnetic film 22, that is tosay, it is in the state of the elliptically polarized light. This lightcontains afresh an s component which is polarized in the directionperpendicular to the sheet surface of the drawing, that is, in the ydirection. The p component and the s component are separated from eachother by a polarization analyzer, or, the polarizing prism 33. The pcomponent travels straight and its most parts return again to the laser11. However, a part of the p component is separated by the beam splitterduring its travel and is converged on the light receiver 34 by a lens330. The s component separated by the polarizing prism 33 as thepolarization analyzer is converged upon the light receiver 35 by a lens331. The output electric signals of the light receivers 34 and 35 areapplied to a differential amplifier 60 and its differential signalappears at the output terminal A.

The magnetic film 22 must be positioned in the proximity of the focalpoint of the laser light converged by the lens 16. Actually, however,the magnetic film 22 tends to deviate from the focal point due to thesurface deviation arising from the rotation of the disc. If the film isout of the focal point, the light quantity of the p component from thereflection to its return to the laser and the light output of the laservaries in accordance with this change. This is a light feed-back effect.The light output of the laser is detected by detecting the lightoutgoing backwardly by means of a light receiver 71 and its output isfed to an electromagnet 73 via a feed-back amplifier 72.

On the other hand, an iron piece 74 is fixed to the focusing lens 16,both of lens and iron piece supported by a spring 75. According to thisconstruction, when the position of the magnetic film 22 deviates fromthe focal point of the lens 16, the excitation current of theelectromagnet 73 changes due to the abovementioned light feed-backeffect so that the electromagnetic attraction to the iron piece 74changes and the lens 16 is pulled back to the correct focal point.Automatic focal adjustment is made in this manner. The electromagnet 73is arranged so as not to magnetize the magnetic film 22.

A so-called tracking technique, which prevents the focused spot of thelaser light from coming out of the information recording track is wellknown in the art of the conventional optical disc technique.

Next, the characteristic and arrangement of the phase plate as one ofthe constituent requirements of this embodiment will be described. Thisphase plate has two principal axes of polarization on the plane crossingat a right angle with each other on the plane pependicular to theoptical path, that is to say, on the x-y plane of FIG. 14. Theorientation of these two principal axes, or, the a axis and the b axis,are shown in FIG. 15. In other words, the x axis (p axis of thepolarizing prism) and the y axis (s axis of the polarizing prism) arearranged so as to be inclined by an angle θ from the a axis and b axis,respectively. Further, it is assumed that there is a phase difference ofa phase angle 2Δ between the a axis and b axis. That is to say, whenrefractive indice in the directions of the a and b axes are n_(a) andn_(b), respectively, 2π(n_(b) -n_(a))d/λ=2Δ, where d is a thickness ofthe phase plate and λ is a wavelength. According to this arrangement,the light passing through the phase plate is converted into theelliptically polarized light (including the circularly polarized light).

Next, the reflection characteristic on the surface of the magnetizedsubstance will be described. When reflection tensor representing themagnetic Kerr effect is ##EQU1## R_(p) and R_(s) represent the Fresnelreflection coefficients and K represents the Kerr reflectioncoefficient. If the imaginary part of K is K.sup.(i), the followingequation represents the Kerr ellipticity:

    E=K.sup.(i) /R.sub.p                                       (1)

Under the abovementioned condition, the intensity I of the lightincident to the light receiver 35 is obtained in the following mannerwith the proviso that the optical axis of the incident light isperpendicular to the reflection surface.

    I=I.sub.r (S.sup.2 -2ES+A)                                 (2)

    S=sin 2θ·sin 2Δ                       (3)

    A=|K/R.sub.P |.sup.2 +C                  (4)

In the abovementioned formulas, I_(r) represents the Fresnel reflectionlight intensity, and A represents the ratio of the received lightintensity to I_(r) when S=0 or when θ=0 or Δ=0, and is a sum of theportion |K/R_(P) |² due to the Kerr effect and other portions C due tothe stray light or to the incompletness of the polarizing prism 33.Equivalently, the noises of the light receiver, dark current andelectronic circuit are contained in C. When the magnetizing directioninverses, K.sup.(i) of the equation (1) or the sign of E inverses andthe peak value of the signal output at that time is given as followsfrom the equation (2);

    I.sub.s =|I.sup.(+) -I.sup.(-)| =|4ESI.sub.r |                                                (5)

Here, I.sup.(±) represents the received light intensity when themagnetization is in the positive or negative direction, respectively.The noise arising from the d.c. background is ##EQU2## Here, ηrepresents fluctuation factor of light intensity received by the lightreceiver 35. In order for the signal to be detected, therefore, thefollowing relation must at least be satisfied from equations (5) and(6);

    4|ES|>ηA

that is,

    |sin2θ·sin 2Δ|>ηA/4|E|          (7)

As the effective value of the light intensity fluctuation factor, theratio between the root mean square value of the fluctuation and the meanvalue of the light intensity may be used. Namely, in the embodimentshown in FIG. 14, equation (7) must be satisfied as one of therequirements when the phase plate of a phase difference 2Δ is arrangedat an azimuth θ.

Next, the optimum points of Δ and θ can be obtained in the followingmanner from the condition which makes the signal-to-noise rationmaximum; ##EQU3## However, the condition of the above equation is notnecessarily essential. If the absolute value of S determined by thisequation is extremely smaller in comparison with 1, the absolute valueof S may be set to be greater than the optimum point given by the aboveequation. For, as can be appreciated from the equation (5), the absolutevalue of the signal output increases with an increasing value of |S|,and it is sometimes more advantageous practically to have a higher leverfor the output signal even at the sacrifice of the signal-to-noise ratioto some extents.

To reduce the noise I_(N), the light intensity fluctuation factor η mustbe reduced, as can be understood from the equation (6). An effectivemethod for this purpose will be described in the paragraph to follow. InFIG. 14, the p component light quantity incident to the light receiver34 contains the noise component having the same phase as that of thesignal of the light receiver 35. Accordingly, the noise contained in thesignal appearing at the output terminal A can be reduced by making equalthe mean values of signal intensity appearing at the two light receivers34 and 35 through adjustment of the transmission factor of reflectionfactor of the optical system or through adjustment of gain of anelectric signal system and obtaining the difference between them by useof a differential amplifier 60. In this manner, the light intensityfluctuation factor can effectively be reduced. Incidentally, the sameeffect can be obtained by connecting a light receiver 71 to thedifferential amplifier 60 instead of using the light receiver 34.

If |S| becomes greater than the optimum point of equation (8), thesignal-to-noise ratio decreases with |S|. In order for the signal to bedetected, |S|must satisfy the following equation (9);

    |sin 2θ·sin 2Δ|<4|E|/η           (9)

Namely, the phase angle 2Δ and aximuth θ of the phase plate must satisfythe condition of the formula (9). Since |S|≦1, however, the condition ofthe formula (9) can be satisfied automatically if the Kerr ellipticity|E| is so great, or if the light quantity change ratio is so small, asto make the right side of equation (9) greater than 1.

As described above, in the optical system shown in FIG. 14, |sin 2θ·sin2Δ| must fall within the range stipulated by equations (7) and (9) whenthe phase difference of the phase plate 90 is 2Δ and the angle describedby its principal axis with the polarizing direction of the incidentlinear polarized light (p polarized light) is θ.

In the embodiment shown in FIG. 14, a rotary polarization element(Faraday rotation element) may be employed in place of the phase plate90. FIG. 16 shows an example of the construction of the rotarypolarization element. This drawing is a sectional view of the rotarypolarization element which is cut on the plane parallel to the sheetsurface of FIG. 14. A magnetic field H is applied by a ring-like magnet82 onto a disc 81 consisting of glass, rare eath, iron, garnet or thelike. The direction of the magnetic field H is parallel to the opticalpath. The linearly polarized light passing through this disc has apolarizing direction which rotates with the optical axis as its axis.This angle, or the Faraday rotation angle φ, varies with the magneticfield, the thickness of the disc 81 and the material of the disc.

When the rotary polarization element is employed, the condition whichpermits the detection of the signal is as follows. Namely, when the realpart of the Kerr reflection factor K is defined as K.sup.(r) and theKerr rotation angle, as

    G=K.sup.(r) /R.sub.p                                       (10)

the following conditions (11) and (12) must be satisfied simultaneously;

    |sin 4φ|>ηA/2|G|(11)

    |tan 2φ|<2|G|/η(12)

In comparison with the phase plate, the rotary polarization element isdisadvantageous in the aspect of the weight of the magnet and in thatmagnetic shield must be considered so as not to allow the magnet 82 toact upon the magnetic film 22. Depending upon the material of themagnetic film 22 and upon the wavelength of the laser, however, itsometimes happens that the Kerr ellipticity E is small whereas the Kerrrotational angle G is large. In such a case, the rotary polarizationelement must be used.

The present invention can be applied to the observation of magneticdomains and to the detection of magnetic bubbles.

In the foregoing embodiments, the change in the ellipticity arising fromthe optical elements other than the optically anisotropic medium as theobject for the measurement can be compensated for by adjusting theellipticity generated by the elliptically polarized light generationmeans.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the magneto-optic memory inwhich information is recorded in accordance with the state ofmagnetization of a magneto-optical medium, and the recorded informationis optically reproduced.

What is claimed is:
 1. The apparatus for detecting magneto-opticalanisotropy consisting of a light source, a magneto-optically anisotropicmedium to which the ray of light from said light source is irradiated, apolarization analyzer to which the ray of light from said medium isincident and a light detector to which the ray of light obtained viasaid polarization analyzer is incident, the improvement wherein saidapparatus is equipped with means for generating ellipticaly polarizedlight on an optical path consisting of said light source, saidmagneto-optically anisotropic medium, said polarization analyzer andsaid light detector, and said means for generating the ellipticallypolarized light are arranged in such a manner that the ray of lighttravelling towards said polarization analyzer remains the ellipticallypolarized light when it is incident to said polarization analyzer,wherein the ray of light emitted by said light source is linearlypolarized light, and said means for generating the ellipticallypolarized light is a phase plate with the phase difference 2Δ and withthe principal axis oriented at an angle θ with respect to thepolarization direction of said linearly polarized light, which ischaracterized by that the quantity |sin 2θ·sin 2Δ| is stipulated withinthe range given by the following formula:

    ηA/4E<|sin 2θ·sin 2Δ|<4|E|/η,

where E is the Kerr ellipticity on a magnetized reflecting surface, A isthe ratio of the received light intensity to the intensity of theFresnel reflection light under the condition of Δ=0 or θ=0, and η is thefluctuation factor of the received light intensity.